1. Field of the Invention
This invention relates to a BCC type hydrogen-absorbing alloy. More particularly, this invention relates to a hydrogen-absorbing alloy which uses a ferroalloy, is therefore advantageous cost-wise, and has excellent hydrogen absorption and desorption characteristics due to a fine structure formed by spinodal decomposition even when the iron component is increased.
2. Description of the Prior Art
As means for storing and transporting hydrogen, a hydrogen-absorbing alloy can absorb a hydrogen gas to a capacity more than about 1,000 times the volume of the alloy itself, and its volume density is equal to, or greater than, that of liquid or solid hydrogen. It has long been known that metals and alloys having a body-centered cubic structure (hereinafter called the "BCC" structure) such as V, Nb, Ta and a Ti-V alloy absorb and store greater amounts of hydrogen than AB.sub.5 type alloys such as LaNi.sub.5 and AB.sub.2 type alloys such as TiMn.sub.2 that have been already put into practical application. This is because the number of hydrogen absorbing sites in the crystal lattice of the BCC structure is large, and the hydrogen absorbing capacity according to calculation is as great as H/M=2.0 (about 4.0 wt % in alloys of Ti or V having an atomic weight of about 50).
A pure vanadium alloy absorbs about 4.0 wt % which is substantially the same as the value calculated from the crystal structure and emits about half at normal temperature and pressure. It is known that Nb and Ta that are elements in the same Group 5A of the Periodic Table exhibit similarly large hydrogen absorbing quantities and hydrogen desorption characteristics.
Pure metals of V, Nb, Ta, etc, are extremely expensive and these metals are not suitable for an industrial application, where a certain amount is necessary, such as a hydrogen tank, an Ni-MH (metallic hydride) cell, and so forth. Therefore, the characteristics of alloys falling within the component range in which they have a BCC structure, such as Ti-V, have been examined. However, in addition to the problems encountered in V, Nb and Ta in that the reaction rate is low and activation is difficult, their BCC alloys involve a new problem in that they only absorb hydrogen at a practical temperature and pressure but that their desorption amount is small. As a result, the alloys having the BCC phase as the main constituent phase have not yet been put into practical application.
Japanese Unexamined Patent Publication (Kokai) No. 2-10659 is one of the prior art references that describe the V-containing alloys described above. This reference teaches the use of a ferrovanadium to which V is added, for example, as the starting material. Japanese Unexamined Patent Publication (Kokai) No. 4-337045 describes a hydrogen-absorbing alloy which is expressed by the general formula Ti.sub.x Cr.sub.2-y-z V.sub.y Fe.sub.z, where 0.5.ltoreq.x.ltoreq.1.2, 2.0&lt;y.ltoreq.1.5, 0&lt;z.ltoreq.0.5 and 0&lt;y+z&lt;2.0.
Though the cost of these ferroalloys is low, they contain Fe as a component. Therefore, those alloy compositions should be developed so that the characteristics do not change or can be improved even when Fe is added afresh as a component.